The present invention relates to a molding machine for mainly press molding a high precision optical element and a method of producing the optical element by using the molding machine.
Recently, in production of an optical element represented by an optical lens, a number of attempts have been made in which the optical element is obtained by press molding through cancellation of a polishing process. Thus, at present, press molding of the optical element is used for mass production by several optical lens makers. In the most efficient of the known methods for press molding optical elements, molten glass material is poured into a die and then, is press molded. However, in this known method, it is difficult to accurately control contraction of glass during cooling of glass and thus, a high precision optical element cannot be produced efficiently. This is because the temperature difference between the melting point of about 1,300.degree. C. at which glass is molten and the transition point at which glass is set is too large. Therefore, an extremely large period is required for cooling glass. Thus, even if a high precision optical element is obtained, this known method is problematical in view of its efficiency. Accordingly, in a general known molding method, a glass blank is preliminarily worked into a certain shape and then, is supplied between molding dies so as to be press molded through heating as described in, for example, Japanese Patent Laid-Open Publication Nos. 58-84134 (1983) and 60-200833 (1985).
Japanese Patent Laid-Open No. 61-26528 (1986) discloses a machine for pressing a lens, which is constituted by an inlet chamber for a glass preform, a heating chamber, a pressing chamber, a gradual cooling chamber and an outlet chamber for the lens. In this known machine, the machine housing as a whole should be adapted to be evacuated to vacuum and a number of molding dies are employed so as to raise production rate. However, this known machine is quire expensive because the machine housing as a whole should be evacuated to vacuum. Furthermore, since a number of the molding dies machined with high precision are employed, production cost of the lens rises considerably.
Meanwhile, Japanese Patent Laid-Open Publication No. 62-292629 (1987) discloses a machine in which processes of heating, pressing and cooling molding dies can be performed in an identical chamber and the molding dies are heated efficiently from above and below. However, productivity of this prior art machine is low due to its intermittent transport of the molding dies. Furthermore, this prior art machine has a drawback that a large uniform heating portion of a heat source cannot be obtained.
Japanese Patent Laid-Open Publication Nos. 62-292636 (1987) and 2-34526 (1990) discloses a machine in molding dies are heated and cooled from one side. Thus, the conventional machine has such problems that the molding dies cannot be heated and cooled efficiently and that since a temperature difference occurs in a vertical direction of the molding dies, lenses having stable performance cannot be obtained.
Furthermore, U.S. Pat. No. 4,913,718 discloses a machine in which a glass blank and molding dies are separately heated on an identical pallet and the glass blank in a high-temperature state is supplied into the molding dies so as to be press molded. However, this known machine is necessarily structurally complicated and expensive due to the need for it to handle glass blanks at high temperatures, etc.
As is seen from the foregoing, the prior art documents do not give detailed descriptions of temperature distribution and uniform heating of the glass blank, the molding dies, the heat source, etc., on which performance of the molded item directly relies.